Could not create the view: Plug-in "com.adobe.flexbuilder.as.editor" was unable to instantiate class "com.adobe.flexbuilder.as.editor.ui.packageexplorer.FlexPackageExplorer".
To resolve this error, start the flashbuilder with -clean option, this will force flash builder to clear the cache, and rebuild it.
Check the path of your flex workspace, If its pointing to some network location, change the flex workspace path to your local hard disk. When you change the flex workspace, flexbuilder will close and relaunch.
Create a new shortcut and edit the path and add -Clean to the end.
"C:\Program Files (x86)\Adobe\Adobe Flash Builder 4.5\FlashBuilder.exe" -clean